Add LimitWriter util

pkg/kubelet/util/ioutils/ioutils.go

@@ -35,3 +35,36 @@ func (w *writeCloserWrapper) Close() error {
 func WriteCloserWrapper(w io.Writer) io.WriteCloser {
 	return &writeCloserWrapper{w}
 }
+
+// LimitWriter is a copy of the standard library ioutils.LimitReader,
+// applied to the writer interface.
+// LimitWriter returns a Writer that writes to w
+// but stops with EOF after n bytes.
+// The underlying implementation is a *LimitedWriter.
+func LimitWriter(w io.Writer, n int64) io.Writer { return &LimitedWriter{w, n} }
+
+// A LimitedWriter writes to W but limits the amount of
+// data returned to just N bytes. Each call to Write
+// updates N to reflect the new amount remaining.
+// Write returns EOF when N <= 0 or when the underlying W returns EOF.
+type LimitedWriter struct {
+	W io.Writer // underlying writer
+	N int64     // max bytes remaining
+}
+
+func (l *LimitedWriter) Write(p []byte) (n int, err error) {
+	if l.N <= 0 {
+		return 0, io.ErrShortWrite
+	}
+	truncated := false
+	if int64(len(p)) > l.N {
+		p = p[0:l.N]
+		truncated = true
+	}
+	n, err = l.W.Write(p)
+	l.N -= int64(n)
+	if err == nil && truncated {
+		err = io.ErrShortWrite
+	}
+	return
+}

pkg/kubelet/util/ioutils/ioutils_test.go

@@ -0,0 +1,93 @@
+/*
+Copyright 2019 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package ioutils
+
+import (
+	"bytes"
+	"fmt"
+	"math/rand"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestLimitWriter(t *testing.T) {
+	r := rand.New(rand.NewSource(1234)) // Fixed source to prevent flakes.
+
+	tests := []struct {
+		inputSize, limit, writeSize int64
+	}{
+		// Single write tests
+		{100, 101, 100},
+		{100, 100, 100},
+		{100, 99, 100},
+		{1, 1, 1},
+		{100, 10, 100},
+		{100, 0, 100},
+		{100, -1, 100},
+		// Multi write tests
+		{100, 101, 10},
+		{100, 100, 10},
+		{100, 99, 10},
+		{100, 10, 10},
+		{100, 0, 10},
+		{100, -1, 10},
+	}
+
+	for _, test := range tests {
+		t.Run(fmt.Sprintf("inputSize=%d limit=%d writes=%d", test.inputSize, test.limit, test.writeSize), func(t *testing.T) {
+			input := make([]byte, test.inputSize)
+			r.Read(input)
+			output := &bytes.Buffer{}
+			w := LimitWriter(output, test.limit)
+
+			var (
+				err     error
+				written int64
+				n       int
+			)
+			for written < test.inputSize && err == nil {
+				n, err = w.Write(input[written : written+test.writeSize])
+				written += int64(n)
+			}
+
+			expectWritten := bounded(0, test.inputSize, test.limit)
+			assert.EqualValues(t, expectWritten, written)
+			if expectWritten <= 0 {
+				assert.Empty(t, output)
+			} else {
+				assert.Equal(t, input[:expectWritten], output.Bytes())
+			}
+
+			if test.limit < test.inputSize {
+				assert.Error(t, err)
+			} else {
+				assert.NoError(t, err)
+			}
+		})
+	}
+}
+
+func bounded(min, val, max int64) int64 {
+	if max < val {
+		val = max
+	}
+	if val < min {
+		val = min
+	}
+	return val
+}